Yes you can achieve high speed sync using the Elinchrom RX 1200 and the PocketWizard Flex system along with their ST4 module. While the exact sync speed will vary depending upon your camera, we were able to get a 1/4000th with the RX600 and a Canon 5D MarkII. For more information about this type of set up follow Tom Bol's blog on HyperSync and the Rangers. (Here's the link to an article on Pocket Wizards site:
http://blog.pocketwizard.com/?p=3216
. Even though it's about battery packs, the same concepts apply.

Mark Astmann
Lighting Product Manager
Manfrotto Distribution USA

de rosten
wrote:

I want to use my nice Elinchrome RX 1200 set for a high speed shoot ( 1/1000 sec. ) ...

I'm used to work with my Speedlite 580 EX II with high speed sync, but I haven't enough light.